Rewrite navbar generation code from first principles

[hadley]

@maelle I got tired of having a bunch of code copied from rmarkdown and then modified over the years. It was hard to understand the logic and challenging to make changes. So I've rewritten it from scratch. Along the way:

• I documented the target field for links
• I discovered that we were using depth to mean two different things (menu and path), causing search to fail pages deeper in the site
• I updated the generated HTML to match latest BS5 recommendations (i.e. using <ul> and <li> instead of bare <div>s
• I learned that BS5 definitely doesn't support submenus and now give a slighter better error message (instead of just generating HTML that looks weird)
• I added some better errors if your navbar code is malformed

[hadley]

Our label was a bit lazy before

[github-actions]

🚀 Deployed on https://664620be965a71d203d7e8bd--pkgdown-dev.netlify.app

[jayhesselberth]

This looks good but the position of the search box is slightly different on the new site — is there a CSS rule that also needs updating?

[maelle]

Nice!

• should there be tests for the two other new errors?
• is it worth re-running an HTML validator?
• should there a "gallery" of some YAML configurations for the navbar in the tests?

[hadley]

• Yes, done!
• Already did it 😄
• I think the existing tests are ok since the structure is now pretty simple given that we are clear are not about supporting submenus. But I'll add a comment to explain a bit more.